The invention relates to induction motors.
The rotors of known induction motors are required to have high permeability magnetic flux paths and high conductivity electric current paths. Accordingly it is usual to construct the rotor from thin steel laminations threaded with copper or aluminum bars. One important reason for laminating the rotor steel is to avoid the high frequency loss caused by pulsation of the air gap flux due to the stator slots.
A necessary design feature of laminated rotor induction motors is that the circular depth of the rotor laminations be of the same order as that of the stator (in order to carry the flux) (FIG. 1).
In certain special applications, where the overall machine diameter is limited, it is not possible to provide the necessary annular depth for the rotor laminations, while providing a shaft of sufficient cross-section for torque transmission, without a substantial derating of the power output of the machine.
Non-laminated rotors have been proposed for highly specialised relatively long machines of relatively small tranverse diamensions. Such machines are in a class of low power machines. The rotor then comprises a homogeneous steel cylinder, or tube, and the rotor material is chosen for its ability to carry magnetic flux and electric current and to transmit torque.
The material usually chose is a low carbon steel which often gives an acceptable compromise of magnetic and electric properties (FIG. 2). However, known motors having solid steel rotors suffer from two major disadvantages: high slip speed (and hence low efficiency) and poor power factor.
To reduce slip speed the rotor surface of known motors is sometimes covered with a high conductivity sleeve of copper. In such motors the steel carries the flux and the copper carries most of the current. However a major limitation to this type of rotor is the presence of high frequency rotor losses which can only be avoided by restrictive stator design. Also, because the high conductivity sleeve is in the airgap of the machine, the magnetic airgap ("entrefer") is increased over that for a homogenous rotor and the power factor is considerably worsened (FIG. 3). Such rotors are therefore proposed only in small induction motors of low output power on in current-driven machines, such as eddy-current couplings.
The present invention has the following objectives for an electric motor such as a submersible induction motor driving a pump, for example:
(1) most robust construction possible
(2) high ratio of starting torque/starting current
(3) minimum axial length (to minimise flexible shaft problems)
(4) best efficiency and power factor compatible with (1), (2) and (3).
Most known submersible pump motors have outside casing diameters in the range 100 to 350 mm. (3.94 inches to 13.78 inches). Axial lengths range from a few meters to 25 m. (984 inches).
Conventional known rotor constructions, involving laminations built up on a shaft, suffer from disadvantages in respect of the objectives:
(a) the laminations do not contribute to the strength of the rotor: this is provided by the shaft;
(b) starting torque/starting current ratios are of the order of 0.5:1 to 0.3:1 due to the natural characteristics of a laminated cage rotor. Increasing the rotor resistance to improve this ratio causes a reduction in full load efficiency and the need to dissipate excess heat from the rotor; (c) because of (a), the laminations do not contribute to shaft rigidity. The shaft diameter must therefore be chosen to ensure an acceptable maximum bending deflection under excited conditions. This means that the radial depth available for the passage of rotor flux is severely limited. (This effect is compounded if there is an additional requirement for the shaft to be hollow). Consequently, in machines of same diameter, the ratio radial depth becomes the limiting factor in achieving an adequate flux/pole and hence limits output per unit axial length.
The present invention combines three aspects of construction: to a solid ferromagnetic rotor; a conducting material; and disposition of conducting material in grooves. These aspects are discussed separately below:
A. Solid Ferromagnetic Rotor
The flux-carrying sections and stress-carrying sections in a non-laminted rotor are not distinct and separate but are common and the rotor throughout its full diametral dimension contributes a rigidity and is available for carrying flux. For a given saturation flux density the flux-per-pole, mean air-gap flux-density, and, hence, torque-per-unit-axial-length is relatively increased. The air-gap flux-density is relatively increased some 20 to 35%, depending on gap/diameter.
Because of the inherent high resistance of homogeneous ferromagnetic material excited with alternating current, an excellent starting torque/starting current ratio can be achieved (typical values in the range 1:1 to 0.7:1).
However a relatively high slip of around 6% is required to produce full load torque under full speed running conditions which causes inefficiency and rotor heating.
The solid ferromagnetic rotor provides rigidity, robustness and a good starting torque/current characteristic but at the low slip end of the torque/speed curve the curve is the wrong shape.
B. Conducting material
The shape of the torque/speed curve of a solid rotor machine is dictated by a dimensionless parameter, the so-called Gamma factor (.GAMMA.), derived in the analysis by Davis et al. (See Proceedings of the Institution of Electrical and Electronic Engineers Volume 124, No. 12, page 1187-1196, 1977). Page 1190 of that paper gives generalised torque/speed curves for solid-rotor, eddy-current couplings and illustrates that a homogeneous ferromagnetic rotor has a .GAMMA. factor in excess of 2 for most of the torque/speed curve. An ideal .GAMMA. factor which provides good starting characteristics is in the range 0.1 to 0.3, depending on specific requirements. The paper describes how a copper faced rotor can be used to achieve these values.
C. Conducting material in grooves
The techniques described in that paper give excellent results in couplings but induction motors in the size range applicable to pump drives for oil-wells, for example, very significant high-frequency losses are caused by negative-sequence fields arising from the interaction of the main air-gap field and the stator slotting pattern at normal running speeds.
In a motor according to the invention these losses are minimised and, at the same time, a .GAMMA. factor of 0.1 to 0.3 is achieved. The conducting material is disposed in axial grooves. The number of grooves can thus be chosen to give the minimum high-frequency loss compatible with the avoidance of parasitic reluctance torques.
It should be noted that the invention is applicable also to inverted machines, in which the wound stator is mounted concentrically inside the rotor. In this case the plated grooves would be angularly spaced about the rotor within the bore of the rotor.
In this specification the term "unitary" means "non-laminated" and the term "plating" means electro-plating or spray plating or a process of deposition of material by which the deposited material is bonded to the substrate onto which it is deposited.